We have hypothesized that for thymocytes the "decision" to undergo positive or negative selection is regulated by the balance between glucocorticoid and T-cell receptor (TCR) signaling. We hav now shown that the thymus itself is a site of steroid biosynthesis, and inhibition of steroid biosynthesis enhances the ability of relatively weak TCR-mediated stimuli to cause thymocyte apoptosis. Furthermore, we have produced transgenic mice that express antisense glucocorticoid receptor cDNA in the thymus. Notably, thymi from these mice are very small, with the primary reduction being in CD4+ CD8+ immature thymocytes. These data strongly support the possibility that endogenous glucocorticoids are required for normal antigen-specific thymocyte maturation. Additional studies aimed at elucidating signaling and effector pathways in the apoptotic response have focused particularly on retinoic acid receptors, cell-cycle regulatory proteins (Rb), and inducible lymphocyte nuclear endonuclease. Retinoic acid receptors may in particular offer potential site of intervention for treatment of disorders involving "inappropriate" apoptosis, since our studies with blood from HIV-infected individuals suggest that oral administration of retinoic acid prevents "spontaneous" apoptosis of PBL. In lymphocytes, the transmembrane tyrosine phosphatase CD45 and the src family member tyrosine kinases are particularly important in sign transduction. We have created site directed mutants of a myristylated intracellular fragment of iCD45 that are either enzymatically inactive, or not membrane-anchored. Neither of the molecules restore function in CD45- T cells, demonstrating the requirement for a membrane anchored tyrosine phosphatase in signaling. We have shown the src family tyrosine kinase (Lck and Fyn) activity in CD45- cells is substantially increased. This was unexpected, since CD45 is thought to dephosphorylate a C-terminal negative regulatory tyrosine (Tyr-505, for Lck) an therefore upregulate tyrosine kinase activity. Tyr-505 is indeed hyperphosphorylated in CD45-T cells, but the activity of Lck is nonetheless increased. It is our goal to understand how the interplay between tyrosine phosphatase and tyrosine kinases regulates this pivotal event.